<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        div{
            display: flex;
            width: 1200px;
            height: 600px;
            background-color: pink;
            align-items: flex-start;
        }
        .one, .three{
            /* 分割剩余空间的份数 */
            /* flex: 1; */
            margin: 0 5px;
            width: 300px;
            height: 200px;
            line-height: 200px;
            background-color: purple;
            font-size: 40px;
            text-align: center;
            color: white;
        }

        .two{
            /* 控制子项自己在侧轴上的排列方式 */
            /* align-self: stretch; */
            margin: 0 5px;
            width: 300px;
            height: 200px;
            line-height: 200px;
            background-color: purple;
            font-size: 40px;
            text-align: center;
            color: white;
            order: -1;
        }
        .three{
            order: -2;
        }
    </style>
</head>
<body>
    <div>
        <span class="one">1</span>
        <span class="two">2</span>
        <span class="three">3</span>
    </div>
</body>
</html>